Along with the development and improvement of modern power communication network, self-healing ring network (SDH) routing is gradually increasing, which causes new problem to optical fibre current differential protection for transmission lines.
Existing widely-used data synchronization technique for current differential protection is echo method (ping-pong method) which assumes that the sending and receiving delays are the same. Thus, once the communication routing changes, the delays of sending and receiving channels may be no longer equal. As a result, the correlative clocks in different substations are no more synchronized, neither are the corresponding data. It will severely affect the reliability of protection system and even lead to mal-tripping in some cases. Thus, clock synchronization or data synchronization with asymmetrical channels is very important to the applications in power system, especially to current differential protection.
JP2004226243A provides a solution for clock/data synchronization using GPS, which will not be influenced by asymmetrical channels. GPS can provide accurate, absolute and synchronized time for all substations in theory. But the GPS signal and GPS device are still not reliable enough for protection applications at present. As a result, GPS have not been widely used by current differential protections for synchronization currently.